The “cost per student” number that you always see isn’t really the cost per student, it is the total budget divided by the number of students. To illastrate this, if you just had 1 student and their $9k the school wouldn’t be able to provide a math teacher, a English teacher, a Chemistry teacher, etc. In fact they wouldn’t be able to provide a single teacher at all for the 1 student.
At the other extreme, if you have a school of 1,000 students happily chugging along educating those 1k students and 1 chooses to take $8k (voucher isn’t 100%) away from their school and take it to the Catholic school, they public is pretty much out that $8k. The school still has to maintain all the teachers it had before, the janitors, the electricity, etc. There is virtually no reduction in cost for that one student leaving. (In Indiana, it is the family of public school students that pays for books and materials, so no reduction in cost for the school there either).
The Catholic school I am most familiar with in Indiana is NOT hurting for money. Last year the had $4 million less in spending than revenue, yet accepted $1.6million in vouchers.
The Catholic families do pay in taxes, absolutely. I have done the math though and the vouchers are way more than they could conceviably pay in. They are more than welcome to put their child into their local public any time they wish, the opposite isn’t necessarraly true.
I only really do know about Indiana, but I have yet to find a Catholic school where the families there aren’t a LOT wealthier than the families of their public school that they would otherwise attend.
Yes, the test scores of the Catholics are MUCH higher than their nearby publics. But, only you control for the socio-economic status of the families, that difference disappears entirely. That holds true here and nationally. The dirty little secret of education is that the public are actually doing a slight (very slight) better job educating when you compare apples to apples. I know that I only used a tiny sample, but the R^2 value for the correlation between poverty and test scores is actually slightly higher for the Catholic schools in my city than the publics.
https://www.washingtonpost.com/